(a) Field of the Invention
This invention relates to epoxy encapsulating compositions which are used to encapsulate semiconductor devices such as integrated circuits, transistors, diodes and other electronic components. The invention more particularly relates to encapsulating compositions which are flash resistant, i.e., resistant to flow from encapsulating molds through openings to the leads of an electronic component or through parting lines at the interface of pieces of the mold.
(b) History of the Prior Art
Epoxy molding compounds have found wide use as encapsulatants of semiconductor devices such as integrated circuits, transistors, diodes and other electronic components. Such encapsulates contain an epoxy resin, a hardener, a catalyst, a mold release agent, usually a filler, usually a flame retardant, usually a colorant and sometimes a coupling agent.
Exemplary formulations of these ingredients are described in U.S. Pat. Nos. 4,042,550 to Tuller et al; 4,034,014 to Curtis, Jr. et al; 3,849,187 to Fetscher et al; 3,862,260 to Sellers et al; 3,789,038 to Curtis et al and 3,280,218 to Partansky et al. Such compositions are generally classified by the type of hardener, with phenol-derived and substituted phenol- derived resin, carboxylic acid anhydride and amine hardeners being most common. Frequently, the molding compounds are highly filled (over 50% filler by weight), but for some applications, compositions containing low percentages of filler or even unfilled compositions may be used.
When encapsulating integrated circuit devices, it is objectionable to have too much of the epoxy encapsulating composition flow out of the mold along the many leads of the device, since this material, called flash, acts as an electrical insulator, making it necessary to clean this material from the leads before the device is used or alternatively to discard the device.
Attempts have been made to reduce flash by appropriate reformulating of the encapsulating composition, e.g. by utilizing certain additives such as fumed silica in the encapsulating composition. Such additives do, in fact, reduce flash to some extent; however, in the absence of proper processing, flash is still not sufficiently reduced and even with some types of processing such as extruders, flash is not reduced as much as desirable.
Such products believed to be commercially made using extruders and a flash retardant, when examined under an electron microscope appear to consist entirely or partially of closely packed, usually jagged, particles rather than particles entirely embedded in a matrix.
It was previously recognized that a differential roll mill with plows could be used with alkyd molding compounds to obtain good high-shear mixing and blending. Either prior to or subsequent to the present invention, it was known that epoxy encapsulating compositions could be mixed on such a mill. It has not, however, been previously recognized that superior flash resistant epoxy compositons could be prepared using both a differential mill with plows and a flash retarding composition as subsequently described.